Part 5 Period 3 Na to Ar: 5.1 Survey of the individual elements of Period 3

A brief
description of the elements that make up Period 3 of the Periodic Table,
namely, Na sodium, Mg magnesium, Al aluminium, Si silicon, P phosphorus,
S sulfur, Cl chlorine and Ar argon. There physical and chemical
properties are summarised below with links to other more detailed notes,
including more notes on Period 3.

Reacts when
heated strongly in air to form a white powder of aluminium
oxide which has a giant ionic structure, (Al3+)2(O2-)3.

4Al(s)
+ 3O2(g)==> 2Al2O3(s)

The above
reaction occurs very rapidly on a freshly cut aluminium
surface, but the microscopic oxide layer inhibits any
further reaction, giving aluminium a 'lower reactivity' than
expected, and its excellent anti-corrosion properties.

Reaction
of oxide with water:

Insoluble,
no reaction but it is an amphoteric oxide and forms salts
with both acids and alkali (see below).

Reaction of
oxide with acids:

It behaves as a
basic oxide dissolving to form the chloride, sulphate and nitrate
salt in the relevant dilute acid.

Al2O3(s)
+ 6HCl(aq)==> 2AlCl3(aq) + 3H2O(l)

Al2O3(s)
+ 3H2SO4(aq)==> Al2(SO4)3(aq)
+ 3H2O(l)

Al2O3(s)
+ 6HNO3(aq)==> 2Al(NO3)3(aq)
+ 3H2O(l)

ionic equation:
Al2O3(s)
+ 6H+(aq)==> 2Al3+(aq)
+ 3H2O(l)

Reaction of
oxide with strong bases/alkalis:

The oxide also
behaves as an acidic oxide by dissolving in strong soluble bases
to form aluminate(III) salts.

e.g. Al2O3(s)
+ 2NaOH(aq) + 3H2O(l)==>
2Na[Al(OH)4](aq)

forming sodium
aluminate(III) with sodium hydroxide.

ionic equation:
Al2O3(s)
+ 2OH-(aq) + 3H2O(l)==>
2[Al(OH)4]-(aq)

Therefore aluminium
oxide is an amphoteric oxide, because of this dual acid-base
behaviour.

Reaction
of element with chlorine:

Burns when
heated strongly in chlorine gas to form the white*
solid aluminium chloride on heating in chlorine
gas.

2Al(s) + 3Cl2(g) ==> 2AlCl3(s)

*
It is often a faint yellow in colour, due to traces of iron
forming iron(III) chloride.

Aluminium
chloride is a curious substance in its behaviour. The solid,
AlCl3, consists of an ionic lattice
of Al3+ ions, each surrounded by six Cl-
ions, BUT on heating, at about 180oC, the thermal
kinetic energy of vibration of the ions in the lattice is
sufficient to cause it break down and sublimation
takes place (s ==> g). In doing so the
co-ordination number of the aluminium changes from six to
four to form the readily vapourised covalent dimer
molecule,
Al2Cl6, shown above.

Reaction
of chloride with water:

With a little
water it rapidly, and exothermically hydrolyses to form
aluminium hydroxide and nasty fumes of hydrogen chloride gas.

AlCl3(s)
+ 3H2O(l)==> Al(OH)3(s)
+ 3HCl(g)

However, if a
large excess of water is rapidly added, a weakly acidic
solution of aluminium chloride is formed, with the minimum of
nasty fumes!

AlCl3(s)
+ aq ==> Al3+(aq) + 3Cl-(aq)

or more
correctly:
AlCl3(s)
+ 6H2O(l)==> [Al(H2O)6]3+(aq)
+ 3Cl-(aq)

The solution
is slightly acidic, because the hexa-aqa aluminium ion can
donate a proton to a water molecule forming the oxonium ion.

[Al(H2O)6]3+(aq)
+ H2O(l)
[Al(H2O)5OH]2+(aq)
+ H3O+(aq)

Reaction
of element with water:

None due to
protective oxide layer.

Reactions of
the hexa-aqua aluminium ion:

It gives a
gelatinous white precipitate with sodium hydroxide or ammonia
solution which displays amphoteric behaviour by dissolving in
excess strong alkali (NaOH(aq), NOT NH3(aq))
and acids.

Three solid
allotropes. Two are crystalline lattices based on S8
molecules (rhombic and monoclinic sulfur). A 3rd form is an unstable dark brown-black polymeric
form called plastic sulphur, formed when boiling sulphur is
poured onto cold water, great fun, but of little use!